Crystal-controlled oscillator



compared with prior practice.

isfying the given frequency requirement.

Patented Dec. 2, 1941 CRYSTAL- CONTROLLED OSCILLATOR Lawrence F.Koerner, Summit, N. J., assignor to 1 Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York Application May3, 1939, Serial No. 271,427

3 Claims.

' This invention relates to a piezoelectric crystal-controlledoscillator in which the immediate electrical environment of the crystalwhich controls and determines the oscillation frequency is so .modifiedfrom prior practice, and so correspondingly modifies the operation ofthe crystalv inrelation to this environment, as to result in certainspecific and desirable variances in the characteristic of the controlledoscillator as .A recent ruling of the Federal Communications Commissionmakes it mandatory for all stations operating in the ultra-highfrequency broadcasting band to have frequency monitors.

The limit of deviationfrom the assigned carrier frequency is :0.01 percent. This requirement and limit tend to induce the use of a monitoroscillator which indicates percentage deviation rather than cycledeviation as is required in the instance of the present low frequencybroadcast band. For practical convenience an important characteristic ofsuch a monitor would be the use in it of a linear percentage frequencyscale. For instance, since this implies a uniforml dial calibration, itwould tend to avoid an undue crowding at the critical portion of thefrequency scale. What is more important, where crystal control is used,as would be desirable and almost necessary foraccurate frequencyindication, it

would tend to makeit. more nearly possible to use a common percentagefrequency dial or scale for a choice of piezoelectric crystal units allnominally having characteristic frequencies sat- This assumes, ofcourse, that the crystal-controlled monitor oscillator is provided withmeans for varying the frequency slightly but to well outside of thelimit of deviation. This variation of frequency might well beaccomplished, and is accomplished in the present invention, by use of aso-called trimming impedance associated with the crystal. I

Not only should there be a uniform scale calibration, as above, andtherefore a linear relation between frequency and the variation of thetrimming impedance by which the frequency is varied, but forcorresponding reasons the variation of frequency by means of suchtrimming impedance should be large enough to so accommodate this choiceof crystal unit without danger of stopping the oscillations.

It is an object of the invention to so electrically condition thecircuits immediately associated with a control piezoelectric crystalthat a linear percentage frequency variation results from operation ofan impedance element corresponding, for example, to the usual trimminginductance or capacitance associated with said crystal. p 1

Another object of the invention is to so electrically condition theimpedance network' comprising the control crystal of thecrystal-controlled oscillator that the characteristic-frequency "of theoscillator may be varied by'said trim ming impedance to a greater extentthan is represented by prior practice without stoppageof theoscillation. Of course it is contemplated as a very desirable objectthat the relatively wide range of frequency variation necessary tosatisfy the second object should, as well, satisfy the linear percentagefrequency variation characteristic of the fir'st'object over as large aportion of the frequency band as possible. Recognizing that whilea'fixed electrical conditioning-of said network for wide [range'frequency variation represents a very important desideratum,optimum'result'smay be achieved by more or less critical choice of saidelectrical conditions to suit, for example, a particular choice ofcrystal. Therefore, it is a subsidiary object of the invention to employfacile means for effectively changing the electrical conditioningof thenetwork within the'frame of the above objects.

' ance, especially a trimming capacitance, a secand impedance, andpreferably an inductance, in a complemental relation to the firstimpedance. For instance, if a shunt trimming condenser is used, aninductance is inserted between one terminus of the crystal and theterminus of the condenser otherwise connected thereto. The size of thisinductance must be determined experimentally as it will vary not onlywith the percentage deviation required but also'with the type ofcrystal, the frequency of the crystal, and the range of the trimmingcondenser. In order to provide an effective-variation of saidinductance, the invention contemplates the use of an adjustablecondenser in shunt to the inductor, or with like result, the choice ofone of a plurality of fixed condensers, thecombination of inductance andcapacitance at the frequency concerned of. course having an inductivecharacteristic.

The nature of the invention and its various features and objects willappear more fully in the detailed description to follow when taken withthe inductor "L1 shunted -by condenser C2.

the accompanying drawing forming a part of this specification.

In the drawing:

Fig. 1 represents a preferred form of the crystal-controlled oscillatorof the inventiion;

Fig. 1A represents the equivalent electrical circuit of the crystal ofFig. 1; and

Figs. 2' and 3 taken together comprise a graphical representation of theoperation and characteristics of the oscillator of Fig. 1 as effected bythe use of the impedance constituting an important element of theinvention.

Per se the crystal controlled oscillator of the invention is merely acrystal controlled oscillator made variable by the usual trimmingimpedance, a shunt trimming condenser in the preferred form of Fig. 1,wherein the percentage frequency change varies linearly with thevariation of the trimming impedance as indicated by the uniformpercentage scale. The rotorshown diagrammatically in Fig. .1, thedisplacement of 'which is a measure of the percentage frequency change,is integral with the rotor of the variable condenser C1 by'means ofwhich, solely, the frequency of the oscillator is varied while under the.control of the-crystal I. Thought of as a monitor oscillator, the wavefrom the-oscillator is used .to combine 'with an incoming carrier waveto .give a zero beat and the setting of the percentage "frequency scaleat that point .may be used to determine the percentage deviation fromthe standard frequency. For instance, the monitor .oscillator itself mayconstitute a standard frequency :source and the standard frequency may,by obvious mechanical adjustment, be made to coincide with the conditionshown in Fig 1 in which said standard frequency occurs fora-zero readingof the percentage deviation dial. The .subsequent change of frequencytoequality with the incoming wave frequency will be indicated inthepercentage deviation scale. Otherwise account may be taken of theparticular point .on the dial corresponding to such standard frequency.

The oscillator, except for the elements shown .in immediate associationwith the .crystal 1| is of a standard type with the grid leak R, vacuumtube VT and tuned plate circuit L2C4. The fre- .quencyisset anddetermined by the natural frequency of the crystal 1 in the customarymanner. That is, the impedance network :in the input oncult of thevacuum tube andcomprising the crystal as one element is distinguished bya frequency-reactance characteristic, mostly contributed by the crystal.itself, which principally deter- .mines the frequency when treated asareactance cooperating with the interelectrodecapacitance of the tube andthe impedance of the-plate circuit which, at the control frequencmhas aninductive characteristic. Because of the exceedingly great frequencydiscrimination displayed -.by this characteristic .curve as .efiectedprincipally .by the crystal, a very close frequency control is impressed.on the oscillator as a whole. The condensers C1 and C3, of whichcondenser C1 has already been mentioned, shunt the input circuit of.thejtube and the circuit comprising the crystal l and the effectiveinductance constituted by Condenser (capacitor) C1 simulates the usualshunt trimming condenser and is a means for varying thef-requencyover asmallrange within the control of the crystal. The condenser 03 functionssimilarly as the variable condenser C1 but'is invariable during thenormal operation of the oscillator. It is used to adjust the zero of thedial at the desired nominal frequency, either by adjustment if capableof adjustment or by substitution otherwise. The convention used in Fig.1 to indicate the character of this condenser is intended to suggest ascrew-driver adjustment as distinguished from a variation in the commonsense of the Word.

The'inductor L1 is the mostimportant element of the invention, incombination, and it serves two purposes, namely, to widen the range offrequency variation by the variable condenser C1 without stopping theoscillations and to insure, over an adequate portion of this range, thatthe variation of frequency follows the linear percentage deviation lawdesired, and as indicated by the dialorscale of Fig. 1.

The range of frequency variation by the variable condenser Cl, when thecoil L1 is used, is adequate to take account of necessary-changesofcrystals that is, is great enough to compensate for deviations fromthe nominal frequency of each of a plurality of available crystals.However, to insure best operation and to avoid too 'greatreliance on thecondenser C3 to adjust to .zero of the dial, it is expedient toeffectively vary the inductance of inductor L1 and this is afunction ofshunt condenser C2 since the combination of these two elements isinductive at the frequency concerned. This adjustment may be achieved byadjustment of the condenser C2 itself or by substitution of one suchcondenser for another.

Of course, .by proper choice of the network re- :actancesthe oscillatorof the invention may be adapted .for any frequency for which crystals.may be used as the control means. Further, a particular network byproper choiceof the crystal, may 'be used at any one of a number ofdifferent crystal harmonics covering a wide frequency range. Experiencehas proved that by this useof harmonics a carrier frequency range from 5to approximately megacycles may be covered by the circuit of theinvention.

'It .has been found in practice that the maximum frequency deviationobtained by a shunt trimming condenser such as condenser C1 alone, thatis, without the use of the inductance of the invention, is not .over.$0.02 per cent andinmost cases 'is '.less than $0.01 per cent..Since'high frequency transmitters in general have a frequency accuracyof $0.025 ,per cent, in order to develop a monitor oscillator of thesefrequencies, in which therefore the variable frequency highly stableoscillator could be made to zero beat for the transmitter frequency,some means had to be developed toincrease the frequency deviation rangeto at .least this figure of $0.025 per cent. It was found 'that'by theuse of the inductance L1 and .by proper adjustment thereof deviationswere possible up to $1 per cent, that is about fifty times .the formerdeviation obtainable.

A .plot .of percentage frequency deviation against capacitance forvarious effective values of .the inductance of element L1, either byvarying the inductance itself or by adjustment or substitution of theshunting capacitance, was

made and it was found that the curves were of the form a: -Kc

Where equals percentage frequency deviation, K. is a constant, C is thecapacitance'of the condenser C1 and n is a number varying with the valueof the positive reactance of the inductor L1, in which :11. varies fromsome value less than 1 for zero inductance to a maximum of about 2.1.Accordingly, to make possible a linear percentage deviation of frequencyby variation of capacitance C1, it is necessary only to effectivelyadjust the inductance until 11. has a unity value. A practicableroutine'for the adjustment, actual or effective, of the inductance andtherefore for setting up the circuit initially to satisfy a givencondition is as follows. The method, which may well be practised byadjusting C2 although an adjustment directly of LI'WOUld also beeffective is cut and try, but fairly simple since it involves only threefrequency measurements, one at the center of the dial and one at eitherend of the dial. The percentage frequency deviation corresponding to thetwo ends of the dial equally spaced from the center should be equal andthe required percentage away from the center setting. Thetrimmercondenser C3 is then adjusted to give the proper absolutefrequency desired with the dial of C1 set at center.

Figs. 2 and 3 together represent an attempt to explain, in terms ofelemental network theory, how a network of the invention may have thecharacteristics demonstrated experimentally for the circuits of theinvention. The Fig. 2 group of curves demonstrate the effect of thetrimming condenser C1 alone, the Fig. 3 group of curves representing theadditive effects of a series inductance of the invention, at least tothe extent of indicating how the range of frequency variation obtainableby varying the capacitance of condenser C1 is increased thereby. Curve Iof Fig. 2 represents the frequencyreactance characteristic of thecrystal alone, that is, without the trimming condenser. The crystal issimulated by the electrical circuit of Fig. 1A. In Fig. 1A the seriesinductance and capacitance of course represent the dynamic state of thecrystal, the condenser in shunt therewith representing the staticimpedance, that is, capacitance of the crystal. This shunt capacitancewould be that measured when the crystal is passive and would be closelysimulated by a condenser using electrodes like the electrodes of thecrystals and having a dielectric whose characteristics as such simulatethose of the crystal. When a crystal is represented in the simple mannerof Fig. 1A the shunt capacitance shown takes into account not only theactual static capacitance of the crystal but also the capacitanceeffectively in series with the crystal because of the spacing betweenthe crystal and its electrodes and which is not shown separately in thesimulated circuit. This curve I demonstrates by its exceedingly greatslope in its inductive region the distinguishing characteristic of acrystal as a means for frequency control, since in mostcrystalcontrolled circuits and in the circuit of Fig. 1, the crystaloperates in this region.

Curve 2 represents the conditions as affected by adding a trimmingcondenser like condenser C1 in shunt to the crystal. It is evident thatthis curve exhibits a greater slope than curve I. This difference givesa measure of the slight coercion of frequency made possible byadjustment of this capacitance. That is, the crystal may be thought ofas being conditioned thereby S as to give greater scope to frequencychange within the limitation that the frequency must accord with a valueon the inductive portion. It is notable that the zero reactance pointsare coincident for the two curves. It is assumed that the capacitancecontemplated by this curve 2 is not great enough to stop oscillation.While the zero reactance point is not changed, the infinite reactancepoint or antiresonant frequency is appreciably changed, that is, it ischanged from frequency f2 to is.

For practical reasons it is not possible to operate at all inductancepoints of either curve I or curve 2. It is assumed, which is quitereasonable, that operation may occur with a reactance indicated by theline shown above and parallel with the zero reactance line. This lineintersects curves I and 2 at points I) and a, respectively. Therefore,the frequency difference corresponding to these two points is a measureof the possible variation of frequency by a shunt trimming capacitance.That is, while the control frequency, where the crystal alone is used,occurs somewhere in the range of frequency from zero resonance frequencyii to the frequency corresponding to point b, and correspondingly whenthe trimming condenser is used the control frequency occurs at the samereactance value somewhere between said frequency f1 and the frequencycorresponding to point a, the frequencydiiference corresponding topoints a and b is alone attributable to the variation of the shunttrimming capacitance.

In the group of curves of Fig. 3, the same literal designations as inthe group of the curves in Fig. 2 are used so far as possible toindicate analogous conditions. As has been already indicated, thesecurves represent the case of the use of .the series inductance of theinvention. Curve 3 assumes a value of shunt trimming capacitance equalto zero, curve 4 a value of shunt trimming capacitance equal to thatused before. Accordingly, curves 3 and 4 correspond to curves I and 2 ofthe Fig. 2 group. The effect of the use of the inductance is to shiftthe zero or series resonance point, as compared with the conditionrepresented by curve I of Fig. 2 from frequency f1 to frequency f1, butdoes not change the conditions as to anti-resonance. When the shunttrimming capacitance is added to make the curve 4, the zero reactancefrequency remains the same but the infinite reactance or anti-resonantfrequency now shifts from frequency f2 (as compared with curve 2 of Fig.2) to is and an additional infinite reactance frequency is is obtained.The oscillator would operate at frequencies corresponding to theinductive portions of the curve near either is or f5 but the seriesinductance is kept sufficiently low to make is too far distant to betuned by the plate circuit of th oscillator. The maximum frequencydeviation has been increased, as is further shown by the intersectionsof the line corresponding to the reactance required for oscillation withthe two curves at d and c, the difference in the correspondingfrequencies being greater than in the case of the curves in Fig. 2 whereno series inductance is assumed to be used. This demonstrates theefficacy of the series inductance in increasing the range of frequencydeviation without stoppage of oscillation, as has been pointed out. Ithas been found possible to multiply this range by a factor of as much as50 by the use of this expedient when the value of the inductance isadjusted to suit the conditions represented by the different values ofshunt trimming capacitance.

It is notable that although the principle of the invention is wellexemplified by the particular choice of reactive elements indicated inFig. 1, and positioned as there indicated, it may be exemplified also byvariant types of circuit networks particularly by making use of thecomplementary character of inductance and capacitance when used inrelatively contrasting circuit positionings. That is, a seriesinductance simulates in character a shunt capacitance, the conversebeing true. The principle of the invention might well be exemplifiedalso in a circuit in which the trimming impedance of the invention mightbe constituted as before by a capacitance, but with a series inductancelike that of Fig. 1 in series with it. In other words, in shunt to thecrystal would be the inductance and capacitance in series, with eitherthe inductance some fixed value determined by the linearity of thefrequency characteristic desired plus a variable condenser, or a fixedcondenser with a variable inductance, such as variometer, in series.

What is claimed is:

1. In a crystal-controlled frequency monitoring system including anelectric discharge device having input and output terminals and apiezoelectric crystal and adapted to generate electrical oscillations offrequencies determined by vibrations of said crystal, afrequency-shifting circuit of two branches connected in parallel betweenthe input terminals of said device, one of said branches solelycomprising said crystal and a positive reactance impedance meansconnected in series, the other of said branches solely comprising avariable negative reactance impedance means, and a scale adapted toindicate the variations of said negative reactance and hence thefrequency variations of said generated oscillations, said positivereactance having such an empirically determined value that when saidvariable negative reactance is varied in equal degree in eitherdirection from the scale point corresponding to a given frequency, asindicated by corresponding equal scale displacements, the correspondingpercentage frequency changes from said given frequency are likewiseequal for a frequency range of substantially :1 per cent of theoperating frequency, that whereby changes of said negative reactanceeffect linear percentage changes of the oscillation frequency.

2. The combination with an electric discharge device having input andoutput terminals for generating electrical oscillations and apiezoelectric crystal for confining said oscillations to vibrationfrequencies of said crystal, of a circuit arrangement for effectingpercentage frequency changes of the oscillation frequency which arelinear with changes of a circuit element comprising a circuit of atleast two parallel branches, connected at its terminals to the inputterminals of the discharge device, one branch of said circuit solelyconsisting of the crystal and an inductive impedance means connected inseries, a second branch consisting solely of a variable capacitivereatance means, other branches of said circuit, if any, beingcapacitively reactive, and a scale adapted to indicate the variations ofcapacitance in said variable capacitive impedance means and hence thefrequency variations of the generated oscillations, the inductance ofsaid inductive impedance means having such a value as empiricallydetermined that when the capacitance of said variable capacitiveimpedance means is varied in equal degree in either direction from thatcorresponding to a given frequency, as indicated by corresponding equalscale displacements, the corresponding percentage frequency variationsfrom said given frequency are likewise equal for a frequency range ofsubstantially :1 per cent of the operating frequency, whereby changes ofsaid reactance may effect linear percentage changes of the oscillationfrequency.

3. The organization specified in claim 2 including a third branch inparallel to said first-mentioned branches, consisting solely of avariable capacitive impedance means and adapted by adjustment of itscapacitance to determine, independently of other means, a givenfrequency corresponding to a given initial scale reading.

LAWRENCE F. KOERNER.

